Testing concrete floors for excess moisture is an important step before applying finishes or coatings. Here is a simple method for doing a basic moisture test on concrete.
What You’ll Need
- Plastic sheeting, duct tape, and utility knife
- Tape measure
- Contractor’s chalk or marker
Step 1: Prepare the Test Area
First, select an area on the bare concrete floor away from the walls or corners. The area should be at least 18″ x 18″.
Use the tape measure and chalk/marker to outline a square test area on the concrete.
Completely cover the area with plastic sheeting. Secure all the edges with duct tape to seal it against the concrete.
Step 2: Wait 24-72 Hours
Leave the test area covered and undisturbed for a minimum of 24 hours, or up to 72 hours.
This allows moisture in the concrete to evaporate and accumulate under the plastic sheeting.
Step 3: Inspect the Plastic Sheet
After 24-72 hours, remove the plastic sheeting.
Closely inspect the side that was against the concrete. Look for signs of moisture accumulation.
If droplets or condensation has formed, it indicates excess moisture is present. The concrete requires more drying time before coating or finishing.
Step 4: Repeat the Test
If the first plastic sheet shows no signs of moisture, tape down fresh plastic sheeting in the same area.
Wait another 24 hours and inspect again. Absence of moisture both times indicates the concrete is sufficiently dry for finishes.
Tips
- Test several areas, especially areas known to be damp.
- Longer test durations give more accurate results.
- Avoid testing spots with direct sunlight or forced airflow.
- Test during similar temperature/humidity conditions as the planned coating work.
- Moisture tests do not guarantee a dust-free surface. Etchants or grinding may still be required.
Performing this simple test can help identify excess moisture problems before they affect the adhesion and performance of concrete finishes. Allow sufficient drying time for best results.
Frequently Asked Questions
What are signs of moisture problems on concrete?
Common signs of excess moisture in concrete include discoloration, efflorescence deposits, blistering or peeling of prior coatings/finishes, and low pH readings. Moisture testing can confirm if moisture is the cause.
How long should new concrete dry before finishing?
New pours require substantial drying time. A general rule is 30 days of curing per inch of slab thickness. So a 4″ slab would need about 120 days before moisture testing.
When is calcium chloride testing needed?
For flooring installations like carpet, wood, or tile, more quantitative calcium chloride testing per ASTM F1869 is advised to measure moisture vapor emission levels. Plastic sheet testing provides only a basic pass/fail screening.
Can I use plastic sheeting under wood flooring?
No, plastic sheets create moisture buildup and rot risks under wood flooring. Appropriate vapor retarders like polyethylene film should be used under wood. Plastic sheeting is only suitable as a temporary test method.
What if my moisture test fails?
If excess moisture is detected, allow more drying time. Dehumidification, increased ventilation, and surface treatments can also help force-dry concrete slabs. Retest until plastic sheeting results are consistently clean.
Conclusion
Performing routine moisture testing on concrete using plastic sheeting provides a simple go/no go evaluation before applying coatings, paints, stains, or other finishes. Allowing adequate drying time for new concrete construction or after chemical removal processes helps avoid failures, delamination, and moisture-related problems affecting the final floor finish. With this easy test method, contractors can identify moisture issues early and take corrective action before aesthetics, performance, and indoor air quality are impacted.